The present invention relates to a multi-terminal power-combining power amplifier for amplifying a plurality of transmission signals and, more particularly, to a multi-terminal power-combining power amplifier suitable for use in an array antenna.
For example, U.S. Pat. No. 4,618,831 discloses a multi-terminal power-combining power amplifier as a power amplifier that is loaded on a satellite for multi-beam communication use. FIG. 1 depicts the basic construction of a multi-terminal power-combining power amplifier 10 that is what is called a multi-port amplifier.
The multi-terminal power-combining power amplifier 10 is comprised of an input-side multi-terminal power combiner 3 formed by a plurality of hybrids, a plurality of main amplifiers 4M, and an output-side multi-terminal power combiner 5 corresponding to the input-side multi-terminal power combiner. As is the case with a 4-input-4-output type depicted in FIG. 2, the input-side multi-terminal power combiner 3 is formed by a combination of plural xcfx80/2 hybrids HB. The output-side multi-terminal power combiner 5 also has the same construction as mentioned above. In general, the relationship between the number P of ports and the number m of stages of the hybrids is P=m2.
The multi-terminal power-combining power amplifier 10 is used in combination with a multi-beam, adaptive array or similar array antenna. In the multi-beam and adaptive array antennas the beam power of each antenna element varies with traffic. At this time, the beam sending power rises to a maximum value during a full traffic period; there is the possibility that sending power of all antenna elements at a maximum centers on one antenna element. For example, assuming that the maximum sending power of one element is 1 W and the number of elements eight, one element may be supplied with power of a maximum of 8 W. Accordingly, the saturation output of the amplifier for each element of the array antenna needs to be designed with the full traffic period in mind. On this account, the use of discrete amplifiers inevitably makes the array antenna bulky.
In contrast thereto, when the multi-terminal power-combining power amplifier 10 is used in the array antenna, even if the traffic varies between beams, the power of the input signal to, for example, an input terminal IP1 is equally divided by the multi-terminal power combiner 3 to all of its output terminals, and the thus divided signals are provided via the main amplifiers 4M to the output-side multi-terminal power combiner 5, wherein they are combined for output to an output terminal OP1 of the same channel as the above-mentioned input terminal; therefore, the amounts of electrical power that are input to the respective main amplifiers 4M are ideally equal to one another at all times. This enables the saturation power of each amplifier to be designed setting the assumed saturation power for the full traffic period to a fraction of the number of terminals. Accordingly, the multi-terminal power-combining power amplifier 10 has the advantage of reducing the saturation power of each amplifier more than in the case of providing a discrete amplifier for each element of the array antenna. Thus the application of the multi-terminal power-combining power amplifier to the array antenna is effective.
As described in Egami, Kawai, et al., xe2x80x9cMulti-terminal power-combining Multi-Beam Transmission System,xe2x80x9d IEICEJ Journal B, Vol.J69-B, No. 2, 1986, February, however, the multi-terminal power-combining power amplifier is required to have the following characteristics. First, the xcfx80/2 hybrids of the multi-terminal power combiner are uniform in electrical characteristic and low-loss; second, the main amplifiers are uniform in electrical characteristic. Variations of these characteristics cause power leakage between output ports of the power amplifier 10. Of such technical problems, it is relatively easy to form xcfx80/2 hybrids with suppressed characteristic variations and with high precision, but it is appreciably difficult to make the electrical characteristics of the main amplifiers uniform.
As described in the paper by Egami et al., assuming that the isolation between the ports of the multi-terminal power-combining power amplifier is 30 dB and that the number of ports is eight, it is necessary that a standard deviation of the gain of the main amplifier be 0.7 dB or less and that a standard deviation of the amount of phase be 5 degrees or less. It is difficult to fabricate and adjust a number of main amplifiers so that they meet such conditions of standard deviations, taking into account device temperature variations and aging.
An object of the present invention is to provide a multi-terminal power-combining power amplifier that permits implementation of a high inter-port isolation regardless of device temperature variations and aging.
According to the present invention, a feed-forward multi-terminal power-combining power amplifier comprises: a multi-terminal power-combining power amplifier provided with input and output ports of a plurality of channels and having main amplifiers that output amplified transmission signals as main signals to the respective channels; and feed-forward amplifier circuits each formed between the input and output ports of one of the plurality of channels and including the corresponding one of the main amplifiers, for removing a distortion component in the main signal from the corresponding main amplifier.
Since distortion of the main amplifier of each channel is removed by the characteristic of the feed-forward amplifier and characteristics of the devices used are made uniform, leakage power between the ports can be reduced.